In recent years, the technology for identifying objects and authenticating individuals has come to be widely used in supply chain and logistics systems, traffic systems, inventory management systems, book management systems, personal verification systems and electronic money systems by using wireless communication media such as RFID tags and contactless smart cards. A system based on wireless communication media uses a transmission/reception device for exchanging signals with the wireless communication media and supplying electric power to the wireless communication media. In such a system, it is advantageous if a single transmission/reception device can communicate with wireless communication media of a number of different kinds.
However, the communication protocol of the wireless communication media may differ from one system to another. Oftentimes, the bandwidth for the communication between the transmission/reception device and the wireless communication media vary from one system to another so that it is desirable for the transmission/reception device to be able to cover a wide frequency range. To meet such a need, it has been proposed to provide a plurality of resistors in the transmission/reception signal path of a transmission/reception antenna and to switch between the resistors so as to expand the frequency range of the transmission/reception device as disclosed in JP 2007-199871.
However, according to this prior art, because the resistors are always present in the transmission/reception signal path of the transmission/reception antenna, some power loss is inevitable, and the power consumption is undesirably great.
Also, if the intensity of the transmission signal that is passed directly to the reception processing unit of the transmission/reception device is greater than that of the normal reception signal, the transmission signal may prevent the processing of the normal reception signal.